Two-layered clad metal sheets are known which have a base layer of a carbon steel clad with a covering layer of, for example, stainless steel, cupro nickel, etc.. The production of such a two-layered clad metal sheet encounters the following problem. Two layers of different metals have different amounts of thermal contraction when the clad sheet is cooled after levelling by a hot leveller. Therefore the clad metal sheet after cooled down to the room temperature exhibits a camber in such a manner that the metal layer of the greater thermal expansion coefficient is disposed on the radially inner side of the clad metal sheet.
More specifically, in ordinary process for rolling thick metal sheets, the hot sheet after the rolling is levelled by a leveller in order to remove any shape defect in the sheet, e.g., center buckle, edge wave, breadthwise camber and lengthwise camber. The hot-levelled sheet is then cooled on a cooling table. Any shape defect which is generated during the cooling is also levelled by a leveller after the cooling, whereby a flat sheet is obtained.
In the case of a two-layered clad steel sheet composed of a base sheet 1A and a covering layer 1B which have different values of thermal expasion coefficient, a breadthwise camber, which is much greater than that experienced by ordinary steel sheet, is caused during the cooling after the hot levelling, as shown in FIG. 22. In case of the two-layered clad sheet, a difference in the value of the thermal expansion coefficient exists between the base layer which is, for example, a carbon steel and a covering layer which is, for example, a stainless steel, as will be seen from FIG. 23. In consequence, two layers of the clad sheet exhibit different amounts of thermal contraction during the cooling down to the room temperature after the hot levelling, resulting in a large breadthwise camber. The amount y of the camber is maximized when the clad sheet has been cooled down to the room temperature. In the case of a two-layered clad sheet composed of a base layer of a carbon steel and a covering layer of a stainless steel, the camber amount y reaches 300 to 400 mm, although this amount y varies depending on the conditions such as the levelling temperature, sheet thickness, sheet width and the clad ratio, i.e., the ratio of the thickness of the covering layer to the total thickness of the clad sheet.
This heavy camber after the hot levelling causes the following inconveniences.
(a) It is difficult to convey the sheet by table rollers, when the sheet has a heavy camber.
(b) The sheet has to experience an impractically large number of passes during a subsequent cold levelling.
(c) If the camber is extremely heavy, it is practically impossible to level the sheet by an ordinary cold leveller.
The present inventors have already proposed, in Japanese Patent Unexamined Patent No. 42122/1984, a method for levelling a two-layered clad sheet, which has been successfully carried out.
According to this proposed method, the layer having the greater thermal expansion coefficient is forcibly cooled before or during the hot levelling so as to create a temperature difference between two layers, and the sheet is levelled in this state, so that the clad sheet shows only a small camber when cooled to the room temperature.
More specifically, referring to FIG. 23, the covering layer exhibits a greater amount of thermal contraction than that of the base layer. Thus, the base layer exhibits a thermal contraction .DELTA..epsilon.c when cooled down from a temperature Tc to the room temperature, whereas the covering layer exhibits the same thermal contraction .DELTA..epsilon.s when cooled to the room temperature from a temperature Ts which is below the temperature Tc. Thus, if the covering layer is cooled down forcibly to the temperature Ts while the base layer is maintained at the temperature Tc and the sheet is levelled in this state, both layers exhibit the same amount of thermal contraction when they are cooled down to the room temperature, thus the generation of the camber after cooling is prevented substantially.
According to this method, a negative camber is generated in the clad sheet immediately after the hot levelling, as a result of a uniformalization of the temperature, i.e., the transfer of heat from the base layer to the covering layer. However, this negative camber is gradually decreased as the temperature is lowered, and a substantially flat state is obtained when the sheet has been cooled down to the room temperature. In consequence the load in the subsequent cold levelling is reduced or, in some cases, eliminates the necessity for the cold levelling.
Actually, however, there are a variety of types of two-layered clad metal sheets requiring this method. These clad metal sheets have different values of thickness, width, clad ratio and the material of the covering layer. This means that the above-mentioned proposed method cannot equally apply to the variety of clad metal sheets.
Moreover the above-mentioned proposed method encounters the following problems.
(a) The clad metal sheet immediately after the hot levelling exhibits a negative large camber. This undesirably impedes the convey of the clad metal sheet by table rollers, with a result that the production efficiency is impaired seriously.
(b) In order to forcibly cool the layer of the greater thermal contraction amount during the hot levelling, it is necessary that the hot leveller must equip a cooling device. The size and the capacity of the cooling device, however, must be small in the hot leveller. Thus, it is very difficult to attain the desired temperature difference (Tc - Ts) between two layers, particularly when the amount of camber is large, as in the case where the levelling temperature the clad ratio and the sheet thickness are high, high and thin respectively.
(c) The steel sheet has a greater tendency of shape defect such as camber than ordinary steel sheet consisting of a single layer, not only during the finish rolling but also during the subsequent convey. Therefore, a longer time is wasted until the hot levelling is commenced, so that the temperature, at which the hot levelling is started, tends to be lowered undesirably. In such a case, the levelling temperature may further come down as a result of the forcible cooling conducted during the hot levelling. Consequently, as the yield stress of the base layer becomes high, it is difficult to impart the desired plastic deformation by the hot levelling. In such a case, the positive camber, that the layer of the greater thermal contraction constitutes the inner side, appears immediately after the hot levelling, and this camber further grows as the sheet is cooled down to the room temperature.
Accordingly, an object of the invention is to provide a method which prevents the camber of a two-layered clad metal sheet at the room temperature.